This invention relates to a silicone rubber composition of the addition reaction curing type loaded with alkaline earth metal carbonate powder and a method for preparing the same.
In the prior art, alkaline earth metal carbonate powder is used in combination with rubbers for the purposes of improving the heat resistance and electrical insulation of silicone rubber and the acid resistance of cured rubber. Most often, the alkaline earth metal carbonate powder is used in millable silicone rubber compositions relying on organic peroxide crosslinking and RTV silicone rubber compositions relying on condensation crosslinking. However, it is not used in silicone rubber compositions relying on addition reaction crosslinking. The reason is that customary alkaline earth metal carbonate powder used as a filler is alkaline and can react with an organohydrogenpolysiloxane contained as the crosslinking agent in silicone rubber compositions of the addition reaction curing type, giving off hydrogen gas. The evolution of hydrogen gas causes the composition to foam during shelf storage or boosts the internal pressure of a container having the composition received therein, with the danger of eventual rupture of the container.
On the other hand, the addition reaction curing type silicone rubber compositions have many advantages over the condensation type RTV silicone rubber compositions. The former do not require a long time for curing as do the latter. The latter suffers from the reversion that a polysiloxane contained therein undergoes cleavage at elevated temperature under sealed conditions, whereas the former does not give rise to the reversion. Therefore, there is a need for an addition reaction curing type silicone rubber composition loaded with alkaline earth metal carbonate powder.
In particular, there is a need for an addition reaction curing type silicone rubber composition loaded with alkaline earth metal carbonate powder for the purpose of improving adhesion when used as an adhesive, which composition remains stable during storage.
Therefore, an object of the invention is to provide an addition reaction curing type silicone rubber composition loaded with alkaline earth metal carbonate powder, which composition has improved storage stability in that it gives off a minimized amount of hydrogen gas during storage. Another object is to provide a method for preparing the same composition.
It has been found that an addition reaction curing type silicone rubber composition having improved storage stability is obtainable using an alkaline earth metal carbonate which has been surface treated substantially with a diorganopolysiloxane. When the resulting silicone rubber composition is used in conjunction with electric and electronic parts for the purpose of providing a seal around the parts, the rubber is also effective for preventing the parts from corrosion by air-borne corrosive gases. This offers a useful means for overcoming the corrosion problem of electric and electronic parts with automotive emissions such as NOx and SOx. The invention is predicated on this finding.
In one aspect, the invention provides an addition reaction curing type silicone rubber composition comprising (A) a diorganopolysiloxane containing at least two alkenyl groups in a molecule, (B) an alkaline earth metal carbonate powder surface treated substantially with a diorganopolysiloxane, (C) an organohydrogenpolysiloxane containing at least two hydrogen atoms each attached to a silicon atom in a molecule, and (D) a platinum group metal catalyst.
In another aspect, the invention provides a method for preparing a silicone rubber composition of the addition reaction curing type, comprising the steps of mixing a portion or all of a diorganopolysiloxane containing at least two alkenyl groups in a molecule with an alkaline earth metal carbonate at an elevated temperature for surface treating the alkaline earth metal carbonate; mixing the mixture with the remainder of the diorganopolysiloxane if the portion thereof is used in the first mixing step; and mixing the mixture with an organohydrogenpolysiloxane containing at least two hydrogen atoms each attached to a silicon atom in a molecule and a platinum group metal catalyst.